1. Field of the Invention
The present invention relates to the field of automotive-vehicle-borne electronics. More particularly it relates to optoelectronic instruments. More specifically, it relates to spectrometry suitable for determining the composition of a fluid.
2. Description of the Related Art
Competition between the various automobile manufacturers leads to an unceasingly renewed pursuit of better operational performance, in terms of both fuel economy and ecological characteristics. In the field of vehicles powered by internal combustion engines, the composition of the fuel has a direct impact on the performance of the engine. Consequently, knowing the precise composition of the fuel allows certain of the operating parameters of the engine to be adjusted to improve combustion and render the vehicle less polluting.
Moreover, this knowledge may also allow detection of mistakes (filling a petrol tank with diesel and vice versa) that could possibly damage the engine, and may allow a warning to be sent to the driver or the ignition to be blocked so as to prevent irreparable damage. Analogously, it allows detection of a fuel that does not meet legal quality standards.
Analogous observations apply to the engine oil, even to the coolant or to other fluids the properties of which influence the operation of the vehicle.
One means of achieving this compositional analysis of a fluid is to use spectrometric technology.
It is recalled that a spectrometer is a measurement instrument intended to determine the absorption of certain wavelengths of the spectrum (generally of light) by a sample to be analyzed. The wavelengths absorbed form peaks in the absorption spectrum and characterize certain molecules or components present in the sample.
As defined in the context of the present invention, an optical spectrometer is therefore mainly composed of a light source, an optical assembly for forming the light beam so as to create a parallel beam able to pass through the sample, a wavelength filter allowing measurement in a certain wavelength range, and a light detector which measures the light intensity received at this wavelength.
Spectrometers working in the ultraviolet, visible and near infrared wavelength ranges are already in day-to-day use in many fields. Among these fields mention may be made of:                agriculture and food production (for example for monitoring the moisture content of cereal grains, the maturity of fruits, the fat contained in certain foods, etc.);        biomedicine (for example for measuring blood sugar level without taking a sample, etc.); and        fuel production (controlling the quality and the composition of the crude oil, controlling the quality of the end products such as gasoline, diesel, etc.).        
All these fields of application use the same type of measurement instrument, only the size and portability characteristics of which vary. Such instruments optionally use various technologies (Fourier transform, filter, monochromator, diffraction system, etc.) and do not operate over a wide temperature range.
This is because, for reasons related to drift in their performance depending on the temperature, they are rarely used in environments that experience large temperature variations.
These are moreover often laboratory-based instruments, or in any case require easy access to their components for maintenance of the detector or of the light source (the lifetime of which is often short, or in any case shorter than the fifteen or so years necessary in the automobile-borne environment). Consequently, they are unsuitable for long-term installation in an environment that is practically inaccessible for maintenance.
Finally, these spectrometers are not subject to demanding unit production costs, and therefore top-of-the-range components are often used, for example tungsten-halogen or filament lamps that are very temperature-stable but which are incompatible with installation in mass-produced automotive vehicles.
It is understood that these questions, of temperature drift, of component reliability and therefore of access for maintenance, and finally of manufacturing cost, make these commercially available spectrometers unsuitable for use in automobiles and comparable environments.
In an application such as that considered in the field of monitoring automobile fluids, it is necessary to use very low unit cost components that are robust over time, so as to guarantee a lasting operation. One solution is to use light emitting diodes (LEDs) as the light source.
In fact, light emitting diodes are very reliable, well-known components that have a very low cost because they are used in very high volumes in a multitude of applications. They are moreover at the present time available in many wavelength versions, allowing their use in a spectral range from 300 nm (near ultraviolet) to 2500 nm (near infrared).
As may be seen in FIG. 1, the light emitting diodes generally have a fairly wide spectrum characterized by a width at half-maximum from 35 nm to more than 100 nm. It is thus possible, by combining a number of diodes having different properties, to create a source having a very wide emission spectrum.
The emission spectrum and optical power properties of the light emitting diodes may vary substantially depending on the current flowing through them and the ambient temperature in which they are used (see FIGS. 1, 2, 3 and 4 for an example of the value measured for a light emitting diode at 650 nm). And yet, interpretation of the measured absorption requires the intensity, at a given wavelength, of the light wave sent through the sample analyzed to be known precisely.
Their use as a light source for spectroscopy, in environments the temperature of which may vary significantly (for example from −40° C. to +105° C. in an automobile), therefore requires innovative solutions in order to compensate for the natural variation in their properties. More generally, these observations relate to all light sources whose performance varies with aging and temperature.
Low-cost spectrometer devices using an LED-based technology are already known. Several devices of this type are currently patented and commercially available.
One of these devices is marketed by Zeltex and described in patent U.S. Pat. No. 6,369,388 B2. It is a portable spectrometer working in the near infrared intended mainly for analyzing the quality of harvested grain. Various applications are envisioned for the device described, ranging from the biomedical field to agriculture and food production and to fuels.
Among the applications proposed for this device, mention is made of measurement of the octane rating of a gasoline using a discrete spectrum obtained by measuring absorption at 14 different wavelengths, this constraint corresponding to a legal quality standard for fuels distributed by gas stations in an American state.
The Zeltex spectrometer uses the same absorption measurements in its various applications, and therefore independently of the sample analyzed.
The range of temperatures of operation delivering a measurement of guaranteed reliability ranges from −20° C. to +55° C. Such an operating range is incompatible with use in an automobile-borne environment for which the sensors must be rated for temperatures lying between −40° C. and +105° C.
Two methods are presented in the same patent U.S. Pat. No. 6,369,388 B2 for compensating for the effect of temperature.
A first method envisions compensating the results of the spectral absorption measurements depending on the temperature measured by a sensor using a preinstalled compensation algorithm. This method does not attempt to prevent temperature-related deformations of or drifts in the components but attempts to correct the measured values using a previously established correction curve.
A second method considered in the context of this Zeltex device uses a self-calibrating spectrometer. This is then a true compensation of the variations caused by temperature and by aging of the components of the series of measurement units, and in particular the light emitting diodes.
In this method, before each spectral measurement is acquired, the measurement cell is emptied, a first measurement is carried out with the light source turned off (“0” light measurement) which characterizes the measurement noise due to the electronics and the detector, which varies with environmental conditions and time. Then a second measurement is carried out with the light turned on, still without a sample in the cell, thus delivering a “100” light measurement, also affected by the environmental conditions of the measurement.
These two successive measurements allow the spectrometer to self calibrate. However, the constraint of having to empty the cell for this procedure is incompatible with a vehicle-borne use, for example, and, as will be readily appreciated, with permanent installation in a fuel tank or in a fuel distribution circuit.
Another device, designed and manufactured by Rikola, relates to a spectrometer intended for use in a laboratory. This spectrometer may also be employed in an environment the temperature of which varies between 5 and 55° C., too narrow with respect to the constraints of the automobile environment.
It measures the absorption at 32 determined wavelengths. To do this, it uses a monochromator wavelength filter, placed on the source side of the spectrometer, and formed by a diffraction system and 32 light emitting diodes (LEDs). These light emitting diodes are placed on chosen points allowing the desired wavelengths to be obtained.
With the aim of better compensating for the effect of temperature variations on the components of the spectrometer, and thus on the quality of the measurements delivered, the Rikola device allows “0” light and “100” light measurements to be carried out without the sample to be analyzed being present. A calibration of the apparatus is thus obtained.
However, a significant temperature variation results in a deformation of the hardware and therefore movement of the light emitting diodes, which modifies the wavelengths created by the diffraction system and light emitting diode assembly.
Consequently, the spectrometer uses a Peltier device to ensure a sufficiently precise measurement in the temperature range considered, from 5° to 55° C., by regulating the temperature of the grating and light emitting diodes about 30° C. This limits the conditions of use that may be envisioned for the spectrometer.
Moreover, and in contrast to the preceding device, the spectrometer is not associated with a processing algorithm for processing the measured spectrum.
A third low-cost spectrometer device has been developed by Sentelligence, and is for example described in document WO 2003 030 621 A2.
This spectrometer, intended to be vehicle-borne, is produced in the form of an integrated component comprising a light source in the form of light emitting diodes, a truncated conical shaped optic placed in contact with the sample to be analyzed, and a detector placed substantially in the plane of the light source.
It allows the absorption to be measured at various wavelengths. It is not a transmission spectrometer as the two other spectrometers mentioned were, but a reflection spectrometer. It therefore uses measurement of the spectrum reflected from said sample subjected to a known light source to characterize the components present in the sample, and not of its absorption spectrum. Such a choice of reflection spectrometer technology is envisioned for relatively opaque products, which absorb the light rays very strongly (soot, etc.).
The light source is formed by combining light emitting diodes each chosen depending on the wavelengths of which the reflection, by the sample to be analyzed, it is desired to measure. Filters are placed on the side of the light source, so as to limit the beam which passes through the sample to be analyzed to a certain optical wavelength band, and the emission intensity level is optionally controlled by a photodiode (reference detector), placed on an optical pathway independent of the sample to be analyzed.
“0” light (light turned off so as to measure electronic noise) and “100” light (light turned on) measurements may be carried out without the sample to be analyzed being removed from the measurement cell.
This is achieved by correcting the intensity measured at the photodiode so as to calculate an intensity that the measurement detector would theoretically receive if the sample were absent.
By allowing the measurement detector to be calibrated by a reference detector not influenced by the sample to be analyzed, and therefore by allowing compensation for any drift in the light source, for example a temperature-dependent drift, this system is compatible with an application in the field of measurements in difficult environments.
However, the cost of such a spectrometer is determined by the number of light emitting diodes installed, which is directly related to the number of wavelengths to be measured. If this number is substantially larger than six, it becomes too expensive for an on-vehicle unit.
In addition, a source created by juxtaposition of many light emitting diodes may no longer be considered as being a point source, thereby causing detector measurement errors (problems with parallax).
Moreover, the Sentelligence device has to be modified at the design stage for each particular application, thereby requiring measurements at certain wavelengths specific to the sample to be analyzed.
These spectrometers do not allow a high-precision spectral measurement to be obtained, such as that required by certain applications in the automotive field, i.e. for example measurement of the absorption at a given wavelength ±5 nm with a precision of 1% of the given absorption value. They also do not allow use over a wide temperature range.
These various restrictions make these spectrometers unsuitable to use in the context of automobile-borne applications such as the measurement of the chemical composition of a fuel or its precise combustion properties.